The Pokemon GO Field Research is known to reward players with the mythical Pokemon Mew upon completing specific milestones. But it seems Mew isn’t the final reward.
According to Reddit user DeadlyHalibut, upon completing the 7th day Field Research streak, he was rewarded with an encounter with Legendary Pokemon Moltres. Moltres was previously a Legendary Pokemon Raid Boss last year.
Just like any other reward, you can save up your Moltres encounter for a later time.